Retatrutide vs Semaglutide vs Tirzepatide: Full Comparison

The weight loss medication landscape is evolving at a remarkable pace. In just a few years, we have moved from semaglutide (the GLP-1 agonist behind Ozempic and Wegovy) to tirzepatide (the dual agonist sold as Mounjaro and Zepbound), and now to retatrutide, a triple-receptor agonist that has produced larger average weight loss in trials than either of its predecessors.

If you are considering weight loss medication, or already taking one and wondering what comes next, this guide offers a thorough, evidence-based comparison of retatrutide vs semaglutide vs tirzepatide. We cover how each drug works, what the clinical trials actually show, side effects, costs, availability in the UK, and crucially, the blood tests you should be having regardless of which medication you use.

The Three Generations of Weight Loss Injections

To understand the difference between these medications, it helps to see them as three generations of the same scientific idea: harnessing gut hormones to control appetite, blood sugar, and metabolism.

Generation 1 — Semaglutide (2017/2021): The breakthrough. Semaglutide mimics a single gut hormone called GLP-1 (glucagon-like peptide-1). Originally developed for type 2 diabetes as Ozempic, it was later approved at a higher dose for weight management as Wegovy. It proved that targeting GLP-1 alone could produce meaningful, sustained weight loss far beyond anything previously achieved with medication.

Generation 2 — Tirzepatide (2022/2023): The upgrade. Tirzepatide acts on two receptors simultaneously: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). This dual mechanism, marketed as Mounjaro for diabetes and Zepbound for weight loss, pushed efficacy significantly higher. The landmark HbA1c reductions and weight loss figures exceeded semaglutide by a substantial margin.

Generation 3 — Retatrutide (investigational): The triple threat. Retatrutide adds a third receptor to the mix: the glucagon receptor. This addition appears to unlock effects that neither semaglutide nor tirzepatide can match, particularly in liver fat reduction and total metabolic improvement. It is manufactured by Eli Lilly and has reported positive results from several Phase 3 TRIUMPH trials (2025–2026); further trials, including a head-to-head comparison with tirzepatide, are ongoing.

How Each Drug Works: Mechanism Comparison

All three are once-weekly injections (semaglutide is also available as a daily tablet). The fundamental difference lies in how many hormonal pathways each one activates.

GLP-1 is the shared foundation across all three drugs. It slows gastric emptying (making you feel full longer), reduces appetite signals in the brain, and stimulates insulin secretion when blood sugar is elevated. This is why all three drugs work for both weight loss and type 2 diabetes.

GIP (added in tirzepatide and retatrutide) works synergistically with GLP-1 to amplify insulin secretion, improve fat metabolism, and enhance satiety. The combination of GLP-1 and GIP appears to produce greater weight loss than GLP-1 alone.

Glucagon receptor activation (unique to retatrutide) is the most intriguing addition. Glucagon is traditionally viewed as a counter-regulatory hormone that raises blood sugar, but when combined with GLP-1 and GIP agonism, its effects on energy expenditure, fat oxidation, and hepatic lipid metabolism become powerful tools. This is likely why retatrutide produces such dramatic liver fat reduction.

Head-to-Head Comparison Table

The following table summarises the key differences based on published clinical trial data and current market information.

Weight Loss Results Compared: What the Trials Show

Semaglutide: The STEP Trials

The STEP 1 trial, published in the New England Journal of Medicine, enrolled 1,961 adults with obesity or overweight with at least one weight-related comorbidity. Participants receiving semaglutide 2.4 mg weekly lost an average of 14.9% of body weight over 68 weeks, compared to 2.4% with placebo. Approximately one-third of participants lost 20% or more of their body weight, a threshold once thought achievable only through bariatric surgery.

Tirzepatide: The SURMOUNT Trials

SURMOUNT-1 showed tirzepatide producing weight loss of 16.0%, 21.4%, and 22.5% at the 5 mg, 10 mg, and 15 mg doses respectively over 72 weeks. The higher doses helped over half of participants achieve a 20% or greater reduction in body weight.

More compellingly, the head-to-head SURMOUNT-5 trial directly compared tirzepatide with semaglutide over 72 weeks. The results, published in the New England Journal of Medicine in 2025, were decisive: tirzepatide achieved 20.2% body weight loss versus 13.7% for semaglutide. That is a 47% greater relative weight loss with tirzepatide. Among participants on tirzepatide, 31.6% lost at least 25% of their body weight, compared to 16.1% on semaglutide.

Retatrutide: The TRIUMPH Programme

Retatrutide raised the bar further. In the Phase 2 trial published in the NEJM in 2023, the highest dose (12 mg) produced 24.2% body weight loss over 48 weeks. The Phase 3 TRIUMPH-4 trial, reported in late 2025, confirmed and exceeded these results: the 12 mg dose achieved 28.7% average body weight loss (approximately 71.2 lbs or 32 kg) at 68 weeks. The 9 mg dose achieved 26.4%. TRIUMPH-4 enrolled people with obesity and knee osteoarthritis; in TRIUMPH-1, the main Phase 3 obesity trial (reported May 2026), the 12 mg dose achieved 28.3% average weight loss at 80 weeks.

To put this in perspective, if you weigh 100 kg (15 stone 10 lbs), retatrutide could potentially reduce your weight to approximately 71 kg (11 stone 2 lbs) in just over a year. This rivals the results of some bariatric surgery procedures.

Liver Fat Reduction: Retatrutide's Biggest Advantage

Perhaps the most striking difference between retatrutide and its predecessors is the effect on liver fat. Metabolic dysfunction-associated steatotic liver disease (MASLD, formerly called NAFLD or fatty liver disease) affects up to one in five people in the UK and is closely linked to obesity, insulin resistance, and cardiovascular disease. Monitoring liver function markers is critical for anyone concerned about metabolic health.

Semaglutide now has Phase 3 liver data: in the ESSENCE trial (NEJM, 2025), MASH resolved in 62.9% of participants on semaglutide versus 34.3% on placebo. The MHRA granted Wegovy a conditional approval for MASH in adults with moderate-to-advanced liver fibrosis in July 2026.

Tirzepatide has shown similar results in a smaller Phase 2 trial. The SYNERGY-NASH Phase 2 trial showed that 62% of participants on the highest dose (15 mg) achieved resolution of MASH (metabolic dysfunction-associated steatohepatitis) at 52 weeks. More than half showed improvement in liver fibrosis. These results were significant enough for Eli Lilly to pursue Phase 3 liver-specific trials.

Retatrutide has produced some of the largest liver fat reductions reported in clinical trials. In a Phase 2a study published in Nature Medicine, the 12 mg dose reduced liver fat by an average of 86% at 48 weeks. Even more remarkably, 93% of participants on the 12 mg dose achieved normal liver fat levels (below 5%). At 24 weeks, the 12 mg group already showed 82.4% liver fat reduction, with 86% achieving normalisation.

This effect is likely driven by retatrutide's glucagon receptor agonism. Glucagon promotes hepatic fat oxidation, essentially instructing the liver to burn its stored fat for energy. When combined with the appetite suppression and metabolic benefits of GLP-1 and GIP, the result is a comprehensive assault on liver fat from multiple angles simultaneously.

For the millions of people in the UK living with fatty liver disease, these results represent a potential step change in treatment. If you are concerned about your liver health, a comprehensive blood test measuring ALT, AST, GGT, and ALP can provide an early warning of liver stress.

Side Effects: How Do They Compare?

All three medications share a common side effect profile dominated by gastrointestinal symptoms, which is expected given their mechanism of action. However, the severity and specific patterns differ.

Gastrointestinal Effects

Nausea, vomiting, diarrhoea, and constipation are the most frequently reported side effects across all three drugs. These symptoms tend to be most pronounced during dose escalation and generally improve as the body adjusts over 4 to 8 weeks.

Semaglutide trials reported nausea in approximately 44% of participants, vomiting in 24%, and diarrhoea in 30%. Discontinuation due to side effects occurred in approximately 7% of participants.

Tirzepatide showed a somewhat better gastrointestinal profile in some trials, with nausea around 31% and vomiting in 12% at the highest dose. In the SYNERGY-NASH trial, 96% of GI adverse events were mild to moderate.

Retatrutide in the TRIUMPH-4 Phase 3 trial showed nausea in 38–43%, vomiting in 20–21%, and diarrhoea in 33–35% depending on dose. Discontinuation rates were higher at 12–18%, partly because some participants discontinued due to what was described as perceived excessive weight loss.

Unique Safety Signals

Each drug has at least one distinctive safety concern:

• Semaglutide: A rare but documented risk of pancreatitis, and a US boxed warning (which also applies to tirzepatide) regarding thyroid C-cell tumours based on animal studies (no confirmed human cases at standard doses). The prescribing information notes that routine thyroid screening for this is of uncertain value.
• Tirzepatide: Higher rates of gallbladder-related events, including gallstones and cholecystitis, which may be related to the rapid rate of weight loss.
• Retatrutide: A new safety signal emerged in the Phase 3 TRIUMPH-4 trial: dysesthesia (abnormal sensations of touch, such as tingling, numbness, or a burning feeling). This occurred in 8.8% of the 9 mg group and 20.9% of the 12 mg group, compared to just 0.7% on placebo. In the Phase 2 trial, skin-sensation side effects were reported in about 7% of participants on retatrutide versus 1% on placebo; across Phase 3 trials, rates have ranged from roughly 2% to 21% depending on dose and trial. These events were generally mild and rarely led to treatment discontinuation, but this will be an important focus in ongoing safety monitoring.

For all three medications, monitoring cholesterol levels, liver enzymes, kidney function, and blood sugar is recommended at regular intervals during treatment.

Availability and Cost in the UK

Semaglutide (Wegovy / Ozempic)

Semaglutide is the most established option in the UK. Wegovy (the weight-management formulation) has been approved by the MHRA and is available on the NHS under strict criteria, though supply has been intermittent. Private prescriptions are widely available through online pharmacies and weight management clinics. Monthly costs range from approximately £130 to £295 depending on dose and provider, with higher doses typically at the upper end.

Tirzepatide (Mounjaro)

Mounjaro received MHRA approval and is available in the UK, though NHS access remains extremely limited. Under the current phased rollout, NHS prescribing started with patients with a BMI of 40 or above and at least four weight-related health conditions, and was extended in June 2026 to a BMI of 35 to 39.9 with at least four of those conditions. Privately, Mounjaro costs have risen significantly following Eli Lilly's September 2025 price increase, with list prices now ranging from £133 to £330 a month depending on dose; private prices vary by provider. At maintenance doses, Mounjaro is generally more expensive than Wegovy.

Retatrutide

Retatrutide is not available anywhere in the world outside of clinical trials. It remains an investigational drug. Several of Eli Lilly's TRIUMPH Phase 3 trials reported positive results between December 2025 and July 2026, and Lilly plans to submit to the FDA in the first quarter of 2027. No EMA or MHRA filing date has been announced, so UK availability depends on a future MHRA decision and NICE appraisal; no date has been set. Pricing has not been announced but is expected to be in a similar or higher range than tirzepatide given its expanded mechanism.

Be cautious of any online sources claiming to sell retatrutide. Any product being sold as retatrutide before regulatory approval is unregulated, unverified, and potentially dangerous.

How the Three Compare on Different Priorities

Which option, if any, suits you is a decision for you and your prescriber, and it is not simply about the highest weight loss percentage. These are the points where the three medicines differ most.

Regardless of which medication you take or are considering, the single most important supporting action you can take is regular blood testing. Weight loss medications affect multiple organ systems, and monitoring ensures both safety and efficacy.

Blood Tests You Should Have with Any Weight Loss Medication

Whether you are on semaglutide, tirzepatide, or eventually retatrutide, regular blood monitoring is essential. These medications cause significant metabolic changes, and blood tests help your doctor ensure those changes are safe and heading in the right direction.

Here are the key biomarkers to monitor:

We recommend testing at baseline (before starting medication), at 3 months, and then every 6 months while on treatment. If your dose changes significantly or you experience side effects, additional testing may be warranted.

Our Core Health 45 blood test covers 45 biomarkers, including blood sugar (HbA1c), liver, kidney, lipid, CRP and several vitamin and mineral markers, giving you and your doctor a comprehensive view of how your body is responding to treatment. For the most detailed picture, including advanced inflammatory markers and a full thyroid panel, our Peak Insights 70 measures 70 biomarkers and is ideal for anyone on long-term weight management medication.

Frequently Asked Questions

Article written February 2026; trial and regulatory information updated September 2026. It will be updated as new trial data becomes available.